Add Hamburger Menu & Loader HOC

[PhillipUg]

Description

Optimize the header to display well on small screens

• Currently, the top navigation is the same for all screen sizes. When seen on small screens the nav items wrap in an uncomfortable way.
• To fix this i modify the header to show the nav items in a dropdown hamburger menu when the screen size is below 768px
• The hamburger button is aligned to the left side of the screen.

Use of the HOC pattern to implement a better loading state

• I create a Loader component as a Higher Order Component which replaces the "Loading..." message that appears as you navigate/refresh the article pages.
• The loading state is now indicated by a spinner animation (Using react-spinners library by David Hu)